In a projection display system utilizing a cathode ray tube (CRT), electron beams emitted from an electron gun change directions according to the positions of vertical and horizontal deflecting coils to thereby illuminate a fluorescent screen of the CRT to thereby produce a pictorial image on the screen. A convergence coil within the CRT is disposed between three electron guns, corresponding to respective Red, Green and Blue (R,G,B) colors, and three deflecting coils, so that three portions of a pictorial image making up the respective Red, Green and Blue colors can be displayed to thereby generate one complete color image.
While the electron beams illuminate the fluorescent screen of the CRT, an inductance value inherent in the deflecting coils varies based on the generation of heat on the deflecting coils. Accordingly, a phenomenon of distorted convergence results and negatively affects the generation of the colored images composed of Red, Blue and Green colors.
In order to prevent the aforementioned problem, the prior art provided for a fan driven to thereby cool the deflecting coils, but this does not completely solve the convergence distortion.
Accordingly, a technique has been developed to correct the convergence of the projection display system by way of a separate convergence correcting apparatus. FIGS. 1 and 2A are schematic drawings of a projection display system having a conventional convergence correcting apparatus. Three CRTs 12, for generating Red (R), Green (G) and Blue (B) pictorial images, are disposed in respectively different positions as illustrated in FIG. 2A in the projection display system 10. In order to correct the distortion of the frontal projection display system 10, a Charge Coupled Device CCD camera 20 has been utilized.
Firstly, a grid-like pictorial image is projected to a screen 30 using the CRTs 12 of the projection display system 10. The CCD camera 20 connected to the projection display system 10 automatically controls the convergence. The CCD camera 20 detects the distortion of the convergence by analyzing the grid-like image displayed on screen 30. The convergence distortion detected by the CCD camera 20 is used to correct the convergence of the CRTs using a correcting apparatus located within the projection display system 10. FIG. 2B illustrates a convergence distortion of the projection display system 10 shown in FIG. 1. The distortion of the convergence illustrated by the disaccord between the continuous line and the dotted line.
However, with the above convergence collection technique, a separate CCD camera must be installed dedicated to convergence correction, thus increasing the manufacturing cost. On top of that, an accurate convergence distortion cannot be detected due to a screen reflection gain of the CCD cameras.